1. Field of the Invention
The present invention relates to a brightness adjusting device capable of adjusting the brightness of a light source by synchronizing the brightness adjusting pulse with a dimming synchronizing signal of a display apparatus, and a liquid crystal display having the same.
2. Description of the Related Art
A cold cathode fluorescent lamp is commonly used as a backlight source for liquid crystal display devices. In these liquid crystal display devices, the brightness of the backlight is adjusted by controlling the average brightness of the cold cathode fluorescent lamp by changing the duty ratio between a turn-on time and a turn-off time of the cold cathode fluorescent lamp and periodically turning on/off the cold cathode fluorescent lamp. If the turn-off time is shorter than the turn-on time, the brightness becomes high. Conversely, if the turn-off time is longer than the turn-on time, the brightness becomes low. The duty ratio between the turn-on time and the turn-off time of the cold cathode fluorescent lamp is adjusted according to the duty ratio of a pulse signal generated by an inverter circuit that drives the cold cathode fluorescent lamp.
The pulse signal (hereinafter, referred to as a brightness adjusting pulse) can be adjusted through various methods. For instance, the pulse signal can be adjusted by using a differential amplifier (operational amplifier).
FIG. 7 is a circuit diagram showing a conventional brightness adjusting pulse generator. A reference voltage output from a reference voltage generator 701 is input into a positive input terminal of the operational amplifier 704 through a resistor 702. An output of the operational amplifier 704 is transferred to an integration circuit 710 configured with a resistor 705 and a condenser 706, and is put in a positive-fedback loop back to the positive input terminal of the operational amplifier 704 through a resistor 703. In addition, a negative-feedback loop is applied from a node (saw) 711 of the integration circuit 710 to the negative input terminal of the operational amplifier 704, so that the output of the operational amplifier 704 is input into the negative input terminal of the operational amplifier 704. Meanwhile, a brightness adjusting voltage output from a brightness adjusting voltage generator 708 is input into the positive input terminal of an operational amplifier 707 through a level shift 709.
FIGS. 8A to 8D are timing charts showing waveforms at each node of the conventional brightness adjusting pulse generator shown in FIG. 7. FIG. 8A shows the voltage waveform of an output node 712 of the operational amplifier 704, FIG. 8B shows the voltage waveforms of a positive input terminal node (REF) 713 of the operational amplifier 704 and the node (saw) 711 of the integration circuit 710, FIG. 8C shows the voltage waveforms of the node (saw) 711 and a positive input terminal node (A-DIMi) 714 of the operational amplifier 707, and FIG. 8D shows the voltage waveform of an output node 715 of the operational amplifier 707.
The pulse output 801 of the operational amplifier 704 shown in FIG. 8A is integrated by means of the integration circuit 710 including the resistor 705 and the condenser 706, and the node (saw) 711 exhibits a triangular voltage waveform 802, which is linearly changed as shown in FIG. 8B. The node (saw) 711 is fed back into the negative input terminal of the operational amplifier 704 and is compared with a reference voltage 803 input into the positive input terminal of the operational amplifier 704. The signal output from the operational amplifier 704 is put in the positive-feedback loop to the resistors 703, and the output voltage 801 of the operational amplifier 704 is reversed whenever the voltage 802 of the node (saw) 711 reaches the voltage 803 of the node (REF) 713, so that the voltage 803 of the node (REF) 713 is also changed. When the output voltage 801 of the operational amplifier 704 is at the high level, the integration circuit 710 charges the condenser 706 through the resistor 705 and the voltage 802 of the node (saw) 711 is increased. Since the output voltage 801 of the operational amplifier 704 is at the high level, the voltage 803 at the node (REF) 713 that is part of the positive feedback loop including the operational amplifier 704 becomes the high level. In this state, if the voltage 802 of the node (saw) 711 exceeds the voltage 803 of the node (saw) 713, the output of the operational amplifier 704 is reversed and the output voltage of the operational amplifier 704 becomes the low level. When this happens, the electric charges in the condenser 706 of the integration circuit 710 are discharged through the resistor 705. The above procedure repeats, producing the triangular waveform at the node (saw) 711.
The brightness adjusting voltage generated by the brightness adjusting voltage generator is a DC voltage having the brightness level adjusted by the user and is input into the positive input terminal of the operational amplifier 707. The operational amplifier 707 compares the voltage 804 of the positive input terminal node (A-DIMi) 714 of the operational amplifier 707 with the voltage 802 of the node (saw) 711, thereby outputting a brightness adjusting pulse voltage 805.
In general, the frequency of the brightness adjusting pulse is synchronized with the vertical driving signal of the display image signal transmitted to the liquid crystal panel, thereby preventing interference noise and brightness distortion. However, this type of conventional circuit cannot be stabilized due to factors such as an imbalance of the condenser 706, a temperature drift, etc. In addition, when the frequency is synchronized with the vertical driving signal, if the frequency of the input dimming synchronization signal is changed, the amplitude of the triangular wave generated from the integration circuit is changed since the time constant, which is used to determine resistance of the integration circuit and the value of the condenser, is fixed. Then, the dimming control voltage, which is a constant DC voltage, is compared with the triangular wave to change the duty ratio of the dimming control pulse, thereby changing the brightness of the backlight.
The vertical driving signal of the display image signal transmitted to the liquid crystal panel is offset from the frequency of the brightness adjusting pulse, generating interference noise. In particular, if the vertical driving signal of the display image signal that is transmitted to the liquid crystal panel is the common multiple or the common measure of the brightness adjusting pulse, even more interference noise is generated.
There are measures to reduce the interference noise. For example, a dimming control device as disclosed in Japanese Patent Unexamined Publication No. 2003-173892 may be used. FIG. 9 is the circuit diagram of the dimming control device.
The dimming control device shown in FIG. 9 includes an integrator configured with a condenser C5 and a transistor Q4 supplying a charge current, a differential circuit having a reset unit (C4, R7 and D1) that discharges the charge voltage according to a dimming synchronization signal, a transistor Q3, a reference voltage generator, an operational amplifier IC2 that compares the charge voltage of the condenser C5 with the reference voltage and then outputs a pulse signal, a feedback unit that integrates the pulse signal using a resistor R10 and a condenser C6 and then feeds the pulse signal back to a control terminal of the transistor Q4, and an output unit that outputs the dimming control pulse which is obtained by comparing the charge voltage of the condenser C5 with the dimming control voltage using the operational amplifier IC1. The charge voltage waveform of the condenser C5 is a serrated waveform having superior linearity and constant amplitude, and the duty ratio of the dimming control pulse output from the output unit is set proportionally to the dimming control voltage. Therefore, even if the frequency of the dimming synchronization signal is changed, brightness distortion caused by a mismatch between the display driving period of the liquid crystal display and the turn on/off period of the backlight is not generated. In addition, since the duty ratio is proportional to the dimming control voltage, the brightness of the backlight can be stabilized.
However, the dimming control device disclosed in Japanese Patent Unexamined Publication No. 2003-173892 represents a problem in that the frequency of the output dimming control pulse corresponds to the frequency of the dimming synchronization signal.
[Patent Document 1] Japanese Patent Unexamined Publication No. 1995-191298.
[Patent Document 2] Japanese Patent Unexamined Publication No. 2003-173892.